Your child will go through lots of changes in puberty. One of the most significant milestones is her first period. Most of the blood and tissue comes out in the first couple of days, but some girls will continue to have bleeding for up to seven days. The amount of bleeding varies. If a girl has a major growth spurt and has grown some underarm hair, periods are likely to be just around the corner. Cycles are usually between 25 and 35 days.